We report a novel method that facilitates sample pretreatment and detection in amino acid analysis by coupling solvent extraction with capillary electrophoresis. Amino acids and peptides were fluorescently labeled, concentrated into an organic solvent, and then separated by capillary zone electrophoresis with fluorescence detection. To achieve this, acetophenone was first employed to dissolve the derivatizing reagent, fluorescamine. The products, which possessed both hydrophilic and hydrophobic moieties, could be extracted and concentrated into the organic phase by suppressing the deprotonation of carboxyl groups, thus enhancing the hydrophobicity of the resulting molecules through pH modification in the aqueous solution. Furthermore, by fine-tuning the pH value, individual amino acids and short peptide molecules could be separated selectively from the sample bulk. This convenient, chemically controllable concentration technique may be useful in sample concentration and purification of biologically related samples such as amino acids and short peptides.